Coherent detection of optical signals is a technique known to improve the spectral efficiency of fiber-optic transmission. [See, e.g., F. Derr, “Coherent Optical QPSK Intradyne System Concept and Digital Receiver Realization,” J.Lightwave Technol., vol. 10, pp. 1290-1296, September 1992; Y. Han and G. Li, “Coherent Optical Communication Using Polarization Multiple-Input-Multiple-Output,” Optics Express, vol. 12, pp. 7527-7534, 2005; A. Leven, N. Kaneda, V. V. Koc, and Y. K. Chen, “Coherent Receivers for Practical Optical Communication Systems,” Optical Fiber Communication Conference, OThK4, 2007; R. Nagarajan, et. al., “Large-Scale Photonic Integrated Circuits,” J.Sel.Top.Quant. Electron., vol. 11, pp. 50-65, January-February 2005; H. Takeuchi, et. al., “Monolithic Integrated Coherent Receiver on InP Substrate,” IEEE Photon.Technol.Lett., vol. 1, pp. 398-400, November 1989; T. L. Koch et. al., “GaInAs/GaInAsP Multiple-Quantum-Well Integrated Heterodyne Receiver,” Electron Lett., vol. 25, pp. 1621-1623, November 1989; and R. J. Deri et. al., “Ultracompact Monolithic Integration of Balanced, Polarization Diversity Photodetectors for Coherent Lightwave Receivers,” IEEE Photon Technol. Lett., vol 4., pp. 1238-1240, November 1992]. This technique allows the detection of information encoded in optical magnitude, phase, and polarization and—when combined with wavelength-division multiplexing (WDM)—facilitates large, information-carrying capacity in a single optical fiber.
Unfortunately contemporary receivers employing coherent detection typically require numerous components and consequently are both complex and costly.